1. Field of the Invention
This invention relates to a gas seal between two parts between which relative rotation takes place (hereinafter referred to as “mutually rotating” or “mutually rotatable” parts).
2. Description of Related Art
A dry gas seal is a gas seal the operation of which does not depend upon the supply of liquid lubricant, but which relies for lubrication upon the gas it is sealing. Dry gas seals have the advantage over liquid lubricated gas seals that there is no contamination of the gas with a lubricating liquid. This can be of particular importance in the food and pharmaceutical industries. A dry gas seal offers a further advantage over a liquid lubricated gas seal when it is used in combination with gas lubricated bearings. By lubricating both the bearings and seal with gas, rather than with lubricating oil, the conventional lubricating oil system of pump, filter and cooler that would otherwise be required can be dispensed with.
Dry gas seals are commonly used in gas compressors in the food, pharmaceutical and petroleum industries. Gases met with in the uses of dry gas seals include air, natural gas, petroleum gas, carbon dioxide and other gases of high purity, such as gases used in anaesthesia.
The above comments regarding dry gas seals, and their common applications, are not intended to limit the scope of the claimed invention. The comments are made by way of explanation only. For example, the hereinafter described and illustrated preferred embodiments of dry gas seals could be used in other applications and/or in conjunction with gases other than those mentioned above.
FIG. 1 is a schematic sectional drawing of the general arrangement of a known dry gas seal assembly. The member A is attached rigidly to the shaft 1. The member B is stationary but is free to slide axially through a casing C. Leakage between a bore in the casing and the spigot of member B is prevented by an O-ring or similar seal D. High pressure of the gas within the casing is denoted by P3 and the lower outer pressure by P1. The purpose of the seal is to limit to an acceptably low value the leakage from P3 to P1 whether the shaft is rotating or stationary. This is accomplished by arranging for the gap h which separates the active faces F1 and F2 of members A and B respectively to be a few micrometers only.
The member B must be free to slide axially to accommodate differential expansion between shaft 1 and casing C. Because there is no possibility of setting member B rigidly to form a gap h of a few micrometers, the gap has to be determined by the balance of the axial forces which act upon the axially free member B.
With reference to FIG. 1, the counterbalance forces which act upon member B from right to left are the force of the pressure P3 acting upon the annular back face F3 of the member B, the force of pressure P1 acting upon the annular face F4, the force of the compression springs at E and a small friction force of indeterminate sign arising from the seal D and also from the means (not shown) used to prevent member B from rotating. The pressure forces do not vary with the axial position of member B and over the few millimetres of axial movement of member B the force of the compression springs B is essentially constant. The outcome is that the counterbalance force acting upon member B from right to left is essentially independent of the axial position of member B and must lie within a narrow band whose width is determined by the friction force of indeterminate sign.
The necessary condition for the gap h to have a specified value in the operation of a seal is illustrated by FIG. 2 in which the separating force acting upon member B from left to right is illustrated by the sloping line 5. The counterbalance force acting on member B from right to left is represented by line 2 or by line 3 in dependence upon the sign of the friction forces whose double magnitude is represented by the separation 4 of lines 2 and 3. These forces are plotted illustratively against the gap h. The specified value of gap h is determined by the intersection of the sloping line 5 with either line 2 or line 3 in dependence upon the direction in which the friction force is acting. It is closer to reality to consider the width 4 to be a band of uncertainty so that all that is specified is that h lies somewhere between the values of h given by the intersection of line 5 with lines 2 and 3. For an intersection to exist, the separating force acting on member B must depend upon the gap h and for gap h to be set stably the separating force must fall as the gap h increases. The enabling matter in the production of a working dry gas seal is the arranging of a separating force which decreases as the gap h increases. But for that arrangement the faces of members A and B would be in dry contact and would be damaged on rotation of the shaft 1 of FIG. 1.
At values of gap h of a few micrometers, the leakage flow of the gas between the active faces F1 and F2 Of FIG. 1 is viscous in nature. Ultimately as gap h is increased viscous flow changes to turbulent flow. Whilst viscous flow persists, and here for simplicity the faces F1 and F2 of FIG. 1 will be taken as being plane parallel, then the way in which the pressure in the gap falls from P3 at R3 to P1 at R1 is independent of the gap h. The continuous line 6 in FIG. 4 illustrates the pressure distribution in the gap h of that condition. The separating force acting upon the member B from left to right is the area integral of the pressure distribution over the active face of member B from R3 to R1.
If the pressure distribution does not change with gap h, it follows that the separating force does not change with gap h. Then there can be no intersection of the separating and counterbalance forces as has been shown to be essential and as is illustrated in FIG. 2. Consequently, the sealing surfaces of the FIG. 1 seal would not in practice, provide a dry gas seal. There has to be an elaboration of the flow between plane faces to produce a separating force which falls as gap h increases.